In a conventional converter of this description, the primary of the feedback transformer is connected in series with a resistance across the collector leads of the two switching transistors between which the primary of an output transformer is inserted, the latter primary having a center tap connected to one terminal of the d-c source whose other terminal is connected to the two emitters. The secondary of the output transformer, also split into two halves, works into a full-wave rectifier which includes a shunt capacitance and preferably a series inductance for smoothing purposes. The load connected to this rectifier may also be of a partly capacitive nature. Such a converter has the advantages of high voltage stability with variable loads as well as compactness and high operating efficiency. One drawback, however, resides in its tendency to draw a very large starting current, which could be on the order of five to ten times its steady-state current, on account of the capacitances present downstream of its output transformer. Thus, the switching transistors must be designed to handle such large currents even though that need exists only for a very short time, i.e. in a transient phase following cut-in.
A conventional solution to this problem, designed to eliminate the need for oversize transistors, consists in the temporary insertion of a large series resistance into the supply circuit of the converter. Since the protective resistance must be removed or short-circuited after the cut-in but has to be reinserted upon every interruption of normal operation, this solution requires either the assistance of an operator or a complex switching circuit. Such a circuit may comprise, for example, a transistor which acts as the protective resistance by operating linearly in the transient phase but becoming saturated (or being short-circuited) in the steady-state mode.